This invention relates to a method of etching materials, such as lithium niobate, useful in optical or surface acoustic wave devices.
Structures which guide optical radiation without substantial loss of energy are of interest for many applications including communications, remote sensing, etc. Such structures are commonly referred to as waveguides and have a region of a first refractive index at least partially surrounded by a region of a second refractive index with the second refractive index being lower than the first refractive index. The refractive index difference provides the guiding mechanism for the optical radiation. The optical energy density is higher in the high refractive index region than it is in the low refractive index region.
Although many materials have been investigated for possible use in optical waveguides and, in fact, silica-based compositions are presently used for optical transmission lines, one material of special interest for waveguides is lithium niobate (LiNbO.sub.3). This birefringent material has relatively large electro-optic coefficients which are of interest in optical devices, such as switches, polarizers, etc. Several techniques have been developed for fabricating waveguides in LiNbO.sub.3. One exemplary method of fabricating waveguides in LiNbO.sub.3 and other materials is described in U.S. Pat. No. 4,284,663, issued Aug. 18, 1981 to Carruthers et al. A transition metal diffusant is deposited on a major surface of the optically transparent material, typically a single crystal, and is then induffused into the crystal by heating to a temperature between 800.degree. C. and 1100.degree. C. for a suitable period of time. The metal diffusant, e.g., titanium, increases the refractive index of the crystal by approximately one percent which is sufficient to guide both polarizations of optical radiation. If an active optical device is desired, second metal may be deposited on the same major surface, after deposition of a suitable dielectric buffer layer, and patterned to form electrodes so that functions such as switching, modulating, polarizing, etc., may be performed.
Of course, optical structures other than waveguides are of interest. For example, optical gratings can be used to select particular frequencies of radiation from a spectrum by either transmitting or reflecting the desired frequencies. Lithium niobate can be used for gratings as well as waveguides. U.S. Pat. No. 4,094,677, issued on Jun. 13, 1978, describes a technique for fabricating gratings by etching features in a LiNbO.sub.3 crystal using HF acid and a patterned gold/chromium mask. Gratings stated to be suitable for use in surface acoustic wave devices were obtained. Chemical etching is thus one technique used to remove material selectively and fabricate features, but other techniques have been developed which selectively remove material. For example, U.S. Pat. No. 4,598,039, issued on Jul. 1, 1986 to Fischer et al., teaches a method of forming features in, e.g., LiNbO.sub.3, by using electromagnetic energy, e.g., to selectively remove material. This technique is useful in forming waveguides as well as gratings. The technique is also useful with other optical materials, including tantalum, niobium, barium, or strontium oxides such as lithium tantalate (LiTaO.sub.3), strontium barium niobate (SrBaNbO.sub.3), and still others.
The removal of material to form features in LiNbO.sub.3 is of interest to those skilled in the art because a large difference in refractive index can be obtained; i.e., the difference between the refractive indices of LiNbO.sub.3 and air is easily obtained. This refractive index difference is generally larger than that obtained by the indiffusion technique previously described although the presence of a passivation layer might reduce the difference. Neither removal technique, at least at the present time, produces the high-quality optical surfaces that are essential in many optical devices to insure low scattering losses from the waveguide. Moreover, the latter removal technique described suffers from the potential drawback that the radiation used may create defects in the LiNbO.sub.3 crystal when energy is absorbed. This, in turn, affects the electro-optic performance of the device and increases the optical propagation loss. The former removal technique described suffers from several drawbacks, including a relatively slow etching rate. A period of 4 to 12 hours, or more, is described as typical with the longer times being required for deeper etching. Additionally, this etch does not isotropically etch the +C and - C regions of the crystal at the same rate. If domains are present, the surface is decorated and scattering centers are present.
Techniques have been developed which separate niobium oxides from other compounds for analytical purposes, such as the analysis of ores. One technique uses fusion with fluxes. The fluxes are typically very strongly acidic or basic. For example, niobium oxide can be separated from the other components in ores by fusion with potassium bisulfate followed by rinsing in, e.g., water, dilute acids such as HNO.sub.3, or complexing organic acids such as tartaric acid. However, fluxes have not been successfully used in fabricating optical devices in niobium oxides such as lithium niobate. This may be because fluxing has been thought of as a dissolution technique rather than an etching technique, or because of a lack of a suitable mask material; i.e., a material that reacts with the flux considerably more slowly than does the lithium niobate and which can be deposited and removed without adversely affecting the quality of the lithium niobate. Due to the chemical similarities of tantalum and niobium, techniques applicable to niobium are generally applicable to tantalum.